The present invention relates to an exchangeable, wettable solid cathode for an electrolytic cell for producing aluminum via the fused salt electrolytic process.
In the electrolytic production of aluminum from aluminum oxide, the latter is dissolved in a fluoride melt which is comprised, for the main part, of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the aluminum itself forming the cathode. Dipping into the melt from above are anodes which are secured to an overhead anode beam and, in the conventional processes, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anodes. This oxygen combines with the carbon of the anodes to form CO.sub.2 and CO.
The electrolytic process in general takes place in a temperature range of about 940.degree.-970.degree. C. During the course of the electrolytic process the electrolyte becomes depleted in aluminum oxide. At a lower concentration of about 1-2 wt.% aluminum oxide in the electrolyte the anode effect occurs, whereby there is a rise in voltage from, for example, 4-4.5 V to 30 V and higher. Then, at the latest, the crust of solidified electrolyte must be broken open and the concentration of aluminum oxide increased by adding more aluminum oxide (alumina).
In the fused salt electrolytic process for making aluminum it is known to employ wettable, solid cathodes. It has been proposed therefore to employ cathodes made of titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and other substances, including mixtures of these substances which may have been sintered together.
Cathodes which are wet by aluminum offer decisive advantages over conventional cells with an interpolar spacing of ca. 6-5.5 cm. The metal precipitated out in the process flows readily as soon as a very thin layer forms on the surface of the cathode facing the anodes. It is possible, therefore, to conduct the precipitated, liquid aluminum away from the gap between the anode and the cathode into a sump situated outside that gap. Due to the fact that the layer of aluminum on the solid cathode is thin, no non-uniformly thick aluminum layer is formed there, which is in strong contrast to the conventional process, as a result of conventional and electromagnetic forces. Consequently the interpolar gap can be reduced without diminution of current density, that is, a much lower consumption of energy per unit metal produced is achieved.
The U.S. Pat. No. 4,243,502, which proposes solid cathodes in the form of exchangeable elements each with at least one connection for the supply of current, provides a considerable improvement over the wettable cathodes which are permanently anchored in the carbon floor of the cell. As the material for wettable cathodes based on hard metals such as, for example, borides, nitrides and carbides of titanium, chromium and hafnium are relatively expensive, the exchangeable solid body cathodes are partly replaced by another suitable material. According to the German, published patent application No. 30 24 172.6 the exchangeable elements are made out of two parts which are made of different materials, an upper part projecting down from the molten electrolyte into the precipitated aluminum, and a lower part situated wholly in the liquid aluminum. These parts are joined together rigidly by mechanical means and are resistant to thermal shock. The upper part, at least in the surface region, is made solely of material which is wet by aluminum. The lower part or its coating is made out of insulating material which is resistant towards liquid aluminum.
Further trials have shown that the high melting points of both types of material make it necessary to employ expensive high temperature technology in the manufacturing process. Consequently only simple and relatively small parts can be made without problem. Furthermore, the brittleness of the materials more than seldom leads to mechanical damage occurring to the exchangeable cathode elements.
It is therefore a principal object of the present invention to develop an exchangeable solid cathode which can be made using simple technology, exhibits a lower degree of brittleness and yet satisfies all the economic and technical requirements of the modern aluminum electrolytic reduction process.